Interaction between a Capacitor Electrolyte and γ-Aluminum Oxide Studied by Fourier Transform Infrared Spectroscopy

Abstract

The interaction between γ-aluminum oxide and an ethylene glycol (EG) based capacitor electrolyte was investigated by Fourier transform infrared (FT-IR) spectroscopy. It was found that only a few ingredients of the electrolyte react with the oxide (azelaic acid, poly(1-vinyl-2-pyrrolidone) (PVP), and phosphoric acid); the others act as pH or conductivity buffers (boric acid, ammonia, and water). The adsorption of azelaic acid and PVP from the electrolyte was studied as a function of temperature, pH, and time, and the result was compared to the adsorption from model solutions of simpler composition. The influence of other components such as phosphoric acid both in the electrolyte and on the aluminum oxide was also investigated, as was the presence of water. At low pH and high temperature (T ≥ 105 °C) the acid formed an ester with EG and this product adsorbed on the oxide surface. The PVP was attached to the adsorbed azelaic acid by hydrophobic interaction, which is pH independent. Ester formation was found to be catalyzed by other electrolyte ingredients like boric acid. At high pH, surface adsorption of azelaic acid occurs through a deprotonated species, which is mainly coordinated through outer-sphere complexation. At high temperature or after a long equilibration time, the surface of the alumina changed, resulting in less adsorption of the organic substances, independent of pH. This change is due to a selective adsorption of phosphate species from the electrolyte, which block active surface sites.